That means the vCenter Server installer cannot find MSI files of existing vCenter Server services. It could be following reasons:

You delete MSI files in “Temp” folder of the profile you used to install vCenter Server.

The account you used to login and install vCenter Server was roaming profile. The profile’s “Temp” folder was automatically deleted when you reboot/logoff the server.

vCenter Server 6.0 for Windows is consist of lot of standalone package. The upgrading process usually uninstall old packages, and then install newer packages. So the failure doesn’t impact to database or inventory data. You can re-initiate the upgrading again.

But you cannot manually uninstall old package since upgrading process brings down vCenter services first then uninstall old packages. If you already uninstalled old packages, the upgrading process will be stuck on bring down vCenter Services stage since some processes may already be removed. For example “vmware-python” it maps to “VMware vCenter Configuration Service”. If you manually uninstalled it before launch upgrading. It removes the service. Upgrading is not able to check status of the service.

Easiest way to get ride of this problem is

Open Registry Editor (regedit) and go to the path: “HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products”You would see lot of keys are there.

The value of “LastUsedSource” is path of MSI file of old vCenter Server installer.
For example my value is “m;1;X:\vcenter-server\packages\”.

Make sure your server has the path mentioned in previous step (My case it’s X:\vcenter-server\packages\) and old MSI files are available in the path. If it’s a CD-ROM letter, you just need mount old vCenter Server image to the drive.

Copy new vCenter Server image to a local folder, uncompresse and launch installer locally.

Now the upgrading process can read original packages on the mentioned path in step 4. It will automatically remove old packages by the old MSI files.

There are two other workarounds. One is modify the value of “LastUsedSource” to reflect a new location of packages. But you still need the old MSI files be there. Another way is delete the key after you find it in step 2. (I never tested this way but it should work as it let vCenter Server installer thinks the server is brand new so installer can override the existing folders)

I also wrote another article for upgrading error on vCenter Server 5.5 for your reference: